Universal ModderTHE INDEPENDENT GUIDE
FIELD GUIDE / AGENT SETUP ENGLISH · SOURCE-REVIEWED OCT 11, 2026

Universal Modder for Claude Code.
From plugin to your first small mod.

Install the plugin, confirm what Claude can actually use, and walk through a source-based Terraria exercise with clear checkpoints.

Independent tutorials for the open-source project. Not affiliated with the upstream maintainers. Source-reviewed; commands and game examples have not been tested by this site.

Install the Claude Code plugin

The Universal Modder Claude Code setup has two steps: register the upstream marketplace, then install its plugin. Enter these slash commands inside Claude Code, one at a time. They are not Bash or PowerShell commands.

/plugin marketplace add rehan-remade/universal-modder
/plugin install universal-modder@universal-modder

The first command makes the repository’s catalog available. The second selects universal-modder from the marketplace with the same name. Read any client installation prompts and start a fresh session if requested.

You still need Git, Python 3.10+ and ffmpeg. Blender is needed for 3D-to-sprite rendering; a fal key is needed for hosted asset generation. Check the shared prerequisites before continuing.

Sources: Upstream installation ↗

Verify the plugin, skills and CLI separately

  1. Plugin: confirm Claude Code reports the installation completed. An added marketplace alone is not an installed plugin.
  2. Skills: ask Claude to locate mod-any-game and explain its safety rules. If it cannot find it, resolve that before editing a game.
  3. CLI: in the shell Claude uses, check the following help commands.
um --help
um scan --help

Success means CLI help appears. It does not prove a game is compatible or that fal is configured. If um is missing, follow the explicit CLI installation route in the same environment as Claude.

Using a clone instead

git clone https://github.com/rehan-remade/universal-modder
cd universal-modder

Start Claude Code inside the clone. The repository keeps Claude skills in .claude/skills; shared instructions are in AGENTS.md, and CLAUDE.md points to those instructions. The clone’s MCP settings are in .mcp.json. A plugin installation may use its own installed location; do not assume it creates a working clone in your current folder.

Your first Claude Code session: inspect before editing

Start with a read-only inventory. The scan accepts a game name or installation directory; --list lists the installations it discovers. Run these in the agent’s shell:

um scan --list
um scan terraria

The Terraria command requires a discoverable local installation. If it cannot resolve your game, pass its real directory, as shown in the scan reference. A scan report supplies candidate routes, not a guarantee that a loader will work.

Suggested prompt

Use mod-any-game for my owned Terraria installation.
Search field notes and run recon. Explain the game version,
tModLoader route, saves, prerequisites and rollback plan.
Propose one small weapon mod. Do not edit saves or game files yet.
Ask before installing a loader, driving input or publishing.

Sources: Scan arguments and behavior ↗

A Terraria weapon walkthrough with checkpoints

The upstream Fal Arsenal example supplies a tModLoader code reference. The sequence below is an exercise based on that source, not a transcript of a successful Claude session.

  1. Confirm the setup. Compare your Terraria and tModLoader versions with the example. Its documented setup uses single-player tModLoader; multiplayer is not tested. Install tModLoader through Steam and make sure it is in your library.
  2. Isolate a test world. Have Claude locate the actual save directory and create a backup before writing to it. Use a disposable world and character.
  3. Choose one small feature. Ask for a single item or weapon with an easy-to-observe behavior. Use the example’s C# item and projectile code as a reference; postpone nuclear explosions and terrain edits.
  4. Build the minimum slice. Ask Claude to identify the correct ModSources folder and explain the build path. For the existing example, copy its FalArsenal/ source folder there, then use Workshop → Develop Mods → FalArsenal → Build + Reload. For your own mod, use its actual project name.
  5. Confirm it loads. Load the disposable world and check that the item appears and behaves as intended. For the unmodified upstream example, its /arsenal chat command grants the example weapons; do not assume your new mod implements that command.
  6. Add the sprite. Choose fal or a configured local image workflow, then match the frame size, orientation and transparency to the game. Inspect the result in-game.
  7. Record and package. Document the exact versions and observed result. Run the package checks, review the contents and ask before sharing.

Sources: Build, install and example limitations ↗

Claude Code setup troubleshooting

The marketplace was added, but the skill is missing

Run the plugin-install step as well. Check the client’s installation result, restart the session if required, then ask Claude to locate the skill. In a clone workflow, start inside the repository and check .claude/skills and CLAUDE.md.

Claude cannot run um

Check um --help in Claude’s shell, not only a separate terminal. Windows and WSL can have different PATH values. Use the installation troubleshooting guide and avoid guessing bootstrap commands.

fal fails after I set a key

The running agent must inherit FAL_KEY. Restart from the configured environment if needed; keep the key out of logs. Check authentication and credit errors before retrying a paid generation.

Can Claude publish a finished mod automatically?

The upstream workflow requires permission before publication and pull requests. Review the package, licensing, credits and verification statement first.

Complete workflow →um CLI reference →